If you get the GSR you're going to have to source a cable B series tranny (which the place you order it from can probably provide at a reduced cost((don't quote me on that)).

I don't think you would be very disappointed in a B16 in a rex,it's light as hell and the B16 will pull it along just fine.The GSR is going to be more powerful,but it cost like $1800-$2000 more,think turbo with that money and you're blowing the GSR away.